perf(deploy): slim the kiosk closure (disable TTS, Qt, docs)

The disk image was ~6.2 GiB of closure, largely desktop/multimedia baggage a
single-purpose Electron kiosk never uses. Cut the clearly-unused stacks:

- services.speechd off → drops speech-dispatcher's espeak-ng + mbrola voices
  (~1 GB text-to-speech). An ATM does not talk.
- v4l-utils built withGUI=false → drops the entire Qt6 stack (~0.5 GB) that only
  backed the qv4l2 GUI; the v4l2-ctl CLI we actually use for the camera stays.
- documentation off (man/info/NixOS manual) — nobody reads them on a kiosk.

Closure 6.2 → 5.0 GiB. The remaining bulk is electron's own runtime (gtk4/
gstreamer/pipewire, unavoidable), mesa+llvm (GPU), and linux-firmware — those
need heavier / riskier work to touch. Distribute the image as .img.zst.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

deploy/nixos/configuration.nix

@@ -7,6 +7,16 @@
   # System basics
   system.stateVersion = "24.05";
 
+  # ── Image slimming (bitspire#70 sizing) ──────────────────────────────
+  # This is a single-purpose Electron kiosk; strip the desktop/multimedia
+  # baggage NixOS pulls in by default so the disk image stays lean.
+  # - speechd: text-to-speech (speech-dispatcher → espeak-ng → mbrola, ~1GB).
+  #   An ATM does not talk.
+  # - documentation: man/info/NixOS manual — no one reads them on a kiosk.
+  services.speechd.enable = lib.mkForce false;
+  documentation.enable = false;
+  documentation.nixos.enable = false;
+
   # Networking
   networking = {
     hostName = "bitspire";
@@ -121,8 +131,10 @@
     # Node.js for the application
     pkgs-unstable.nodejs_22
 
-    # Camera support
-    v4l-utils
+    # Camera support. v4l-utils' default build drags in the whole Qt6 stack
+    # for its qv4l2 GUI (~0.5GB) — we only ever use the v4l2-ctl CLI, so drop
+    # the GUI.
+    (v4l-utils.override { withGUI = false; })
     fswebcam
 
     # ATM operations